1. Field of the Invention
The present invention relates to an X-ray mask and, more particularly, to an X-ray absorber in an X-ray mask and a method for manufacturing the same.
2. Discussion of the Related Art
The optical lithography exposure technique, one of the leading techniques used currently in the semiconductor industry, has almost reached its technical and economical limitations. As an alternative to the optical lithography exposure technique, the X-ray lithography technique has recently begun to attract much attention. The development of X-ray masks is one of the most important factors needed for the X-ray lithography process.
A general X-ray mask, as shown in FIG. 1, includes a membrane 1 made of silicon nitride (SiN) or silicon carbide (SiC), an X-ray absorber pattern 2 made of a heavy metal such as tungsten (W) or tantalum (Ta) having a high absorption of X-ray on a predetermined portion of the membrane 1, and a silicon substrate 3 formed under the membrane 1, of which the bottom surface opposite to the X-ray absorber pattern 2 is exposed.
At this time, since the X-ray absorber 2 absorbs X-rays, it has to be stable with regard to X-rays. Moreover, its pattern displacement rarely happens despite heat energy generated by X-ray absorption. The reason why pattern displacement is caused is that the absorption of X-ray causes the X-ray absorber to be distorted. That is to say, the absorption of X-ray changes the residual stress of the X-ray absorber. Accordingly, in order to prevent the generation of this problem, the residual stress which is generated when the X-ray absorber is formed should be reduced.
In one of the methods for reducing residual stress of an X-ray absorber, a 10 nm thick .alpha.-W seed layer is formed by evaporation, and then tungsten is sputtered thereon when the temperature of the substrate is 200.degree. C., thus forming an X-ray absorber having low stress.
In another method, tungsten is deposited with a sputtering technique and then a rapid thermal annealing process is carried out over the tungsten, thereby relieving residual stress.
In still another method, tungsten is first deposited. Onto the surface of the X-ray absorber, there is performed an ion implantation process with an inert gas such as neon (Ne), argon (Ar), or Krypton (Kr), thereby relieving residual stress.
In still another method, after tungsten nitride (WN) and tungsten titanium (WTi) in an amorphous phase are deposited thereon, the composite is annealed at a temperature of 300-400.degree. C.
However, conventional methods for manufacturing an X-ray absorber in an X-ray mask having low stress have the following problems.
First, since the annealing process and the ion implantation process are required to adjust the residual stress of an X-ray absorber, the overall manufacturing process becomes complex and an X-ray mask needing a micro dimension pattern is polluted.
Second, since the fine structure of the X-ray absorber is a crystalline phase, the value of its surface roughness becomes about 6-10 nm according to the size of the crystal grains. Accordingly, the micro X-ray absorber pattern has a bad influence on patterning and etching the X-ray absorber.
Third, even though the absorber having an amorphous structure has no grain boundary so that its stress is not changed and the characteristic of the surface roughness is sufficiently excellent to make up for the defects of the absorber having a crystal structure, the difference in stress changes caused by annealing is too small to adjust its residual stress.